Steam pressure vessel resistant to caustic cracking



March 1942- N. B. PILLING El AL 2,275,464

STEAM PRESSURE VESSEL RESISTANT TO CAUSTIC CRACKING Filed April 29, 1958 I Patented Mar. 10, 1942 STEAM PRESSURE VESSEL RESISTANT TO CAUSTIC CRACKING Norman B. Pilling,

International Nickel N. Y., a. corporation of Westfield, N. J and Frederick Straub, Champaign,

111., assignors to The mpany, Inc., New York, Delaware Application April 29, 1938, Serial No. 204,986

8 Claims. (Cl. 122- -4) The present invention relates to the production of high temperature steam pressure vessels resistant to caustic embrittlement and, more particularly, to steam boilers constructed of specially treated nickel steels of particular composition and constitution to render the same resistant to caustic embrittlement and cracking.

In the generation and application of steam, the continuing trend of the art has been toward higher and. higher operating temperatures and pressures in order to take advantage of higher efficiencies attainable under such conditions. Due to the higher stresses in boiler plates, tubes, and the like in high pressure steam generating units, it has been necessary to match the increase in temperature and pressure with stronger structures in the more highly stressed parts. The increased strength could be obtained by using thicker boiler plates and tubes, or by using steels having higher physical properties. In locomotive and marine boilers it has been particularly desirable to keep the weight to a minimum, and as a result engineers and designers turned to alloy steels as a means of getting struc tural parts of adequate strength without adding appreciably to their weight. Experience proved, however, that the resistance of steels to caustic attack was not governed by their strength.

It is well known in the art that caustic embrittlement or hydrogen embrittlement has been a source of failure in pressure vessels, such as stationary steam boilers, railway locomotive boilers, marine boilers, and the like. While this type of failure has occurred in boiler parts that were under normal stresses, caustic embrittlement has generally been especially severe in regions where stresses were magnified or localized, for example in rivets, along riveted joints, welded seams and at other structural discontinuities. has often been that a pressure vessel failed with disruptive force'at a place of localized embrittlement long before the metal in normally stressed parts was appreciably corroded. Not only were such failures a source of danger to life and damage to other property but they entailed enormous The result causticity in the boiler water.

has been known to be greatly aggravated. While some feed waters have been amenable to chemical treatment, such treatment, as is well known. has only been a palliative. In the chemical treatment, sodium carbonate has been used as a chemical modifier of certain types of feed water and has caused the development of excessive.

While periodic blow offs have been helpful in reducing this causticity, nevertheless, it has not been eliminated. Chemical modification of the feed water has not been satisfactory or practicableunder certain conditions, particularly in the case of locomotive and marine steam boilers which had to be supplied with water, even if unsuitable, wherever feed water could be obtained.

The problem of caustic embrittlement has been attacked from another point of view. Elimination of embrittlement, or at least reduction in the rate of embrittlement has been sought through the medium of the materials of which the boiler plates, water tubes, and the like were fabricated.- Many attempts have been made to develop steels which by virtue of their strength,

their composition, their corrosion resistance or freedom from aging changes at boiler operating temperatures were free from caustic embrittlement. I

Although the problem of caustic embrittlement has confronted the art for many years and many attempts have been made to solve this important" yet vexatious problem, none, so far as we are aware, has been wholly satisfactory and successful when carried into commercial practice on an industrial scale.

We have discovered a pressure vessel, particularly. a steam boiler, constructed of specially treated steel having a special composition and constitution which is resistant to caustic embrittlement in use, which is satisfactory and accept able for manufacturing and fabricating such vessels, and which is capable of being readily apeconomic losses in repairs or abandonment of equipment long before it became obsolete. 'Due to its importance, the nature and cause of caustic embrittlementand attempts to control it provoked extensive research, exhaustive experiments and have resulted in voluminous discussion in ;he technical literature for many years.

One line of attack was to treat the boiler feed water with certain chemicals as in the presence at certain natural waters, caustic embrittlement plied to commercial practice.

It is an object of the present invention to provide pressure vessels, particularly steam boilers, practicallyimmune to caustic embrittlement.

It isanother object of the present invention to provide a special steel for the construction of pressure vessels which is not subject to that type of corrosion known to the art as caustic embrittlement.

It is a further object of the present invention to provide nickel alloy steels of particular composition for the fabrication of pressure vessels, particularly steam boilers, which steels dispersion.

It is further within the contemplation of the present invention to provide a steam pressure vessel highly resistant to caustic types of embrittlement made, at least in part, from a low carbon steel which is substantially free from manifestations of blue brittleness, which has a special and critical composition capable of hardening by the dispersion of a metallic constituent containing an alloying element, and which has been treated thermally to place at least a portion of the metallic constituent in a condition permitting it to precipitate under slight plastic strain at temperatures within the range ofabout 360 to 550 F.

Other objects and advantages of the invention will become apparent to those skilled in the art from the following description, taken in conjunction with the drawing in which:

Fig. 1 is a somewhat diagrammatic side elevational view of a'steam boiler with the walls broken away toreveal the internal structure; and 1 Fig. 2 is a'graphical representation of stressstr'ain curves of two steels stressed at elevated temperature, the said steels having substantially the same composition but having been subjected to different heat treatments.

of blue brittleness, and have a metallic constituent containing an alloying element which upon heat treatment is capable of forming, a dispersfible phase having a critical stability at boiler operating temperatures, and which have been heat treated to place at least a part of this dispersive in a condition which will permit it to precipitate under slight plastic strain at temperatures of about 360 F. to about 550 F.

Manifestations of blue brittleness are common in steels of widely varying compositions. A significant characteristic of such steels is that the tensile strength is higher within the temper-' TABLE I Composition g Heat treatment 0 Mn Si Ni 01' Percent Percent Percent Percent 0. 59 0. 16 As rolled.

0.66 0. 65 1650" F., A. C.

1. 16 0. As rolled.

' A. C.=ai'r cooled.

TABLE II Tensile stbength Percent reduction of area Percent elongation R. T. 400 F. 475 F. 550 F. R. T. 400 F. 475 F. 550 F. R. T. 400 F. 475 F. 550 F.

1.. 68.4 74. 8 74. 6 77. 5 57. 4 45. 6 44. 5 42. 5 33. 5 18.5 I 18.8 20.0 2 73. 9 77. 3 77.8 a 79. 7 61.4 52. 3 49. l 49. 4 33. 5 23. 0 2i. 0 20.0 3 73. 0 80.7 80. 7 62.0 52.0 i. 52. 5 33. 7 24.0 20.0 4 72.0 73.0 77. 3 80. 3 72. 5 63. 5 58. 5 56. 2 36. 8 25.0 22. 5 23. 3 5..." 79. 9 80. 5 83. 8 85. l 59. 3 50. 3 48.0 45. 8 33. 8 23. 7 21.0 21. 0

Thousand pounds per square inch.

I, R. T.=room temperature. Referring to the drawing, the reference numeral i represents the masonry of a steam boiler of conventional design. Mounted within the boiler is an inclined bank of tubes designated generally by reference numeral 2. These tubes are arranged in vertical rows and each row is expanded at each end into a continuous header 3, in accordance with customery practice. The headers '3 communicate through connections 4 and 5 with the ends of asteam and water drum 6 located above the bank of tubes 2. The con 'struction of the boiler is well understood *by thoseskilled in the art. The metal parts, particularly stressed parts,',are fabricated from the specially treated steel of a special composition and properties described in detail hereinafter.

Generally speaking, a material particularly useful for the purpose of the present inventiom'm consists of low carbon nickel-containing steel having a dispersible or incompletely dispersed phase of a metallic constituent containing an alloying element. Those steels which have given satisfactory results are free from manifestations which have been found satisfactory inresisting.

caustic embrittlement, the manifestations 'of blue brittleness canbe eliminated by any conventional treatment, such as by through deoxidation of the molten metal with aluminum, titanium, or other.

reactive elements, prior to teeming. In the case of steels treated with aluminum, as an example of the reactive elements used to overcome blue brittleness, a residual aluminum content of at least about 0.015%. assures a steel free from manifestations of blue brittleness. This fact en ables the use of a chemical analysis of such 'a steel as a test to determine the presence or absenceof blue brittleness, since the presence of at least about 0.015% residual aluminum may be taken as a positive indication that the steel is free from blue brittleness.

Typical examples of steels that have been found resistant to caustic cracking and which do not exhibit the bluebrittie phenomenon after thorough deoxidation are listed'in Table III.

Taste Composition Heat treatment Si Ni can A. C.=air cooled.

The effect of heating within the range of boiler ment, however, is not assured merely by constructing the boiler of steel thatis capable of being precipitation hardened, or that has been hardened, by dispersion of a metallicconstituent containing an alloying element. It is essential that the steel be freeof blue brittle" charac-- teristics and also contain an alloying element operating temperatures on the strength and ducv tility of steels 6 and 8 is set forth in'Table IV.

that is, or forms part 01', a dispersible metallic component which is present in an amount moderately in excess of its low temperature limit of solubility, which can be put in or out of solution by heat treatment, and which strengthens the steel without impairment of its ductility. The dispersible metallic component must be partially or wholly in solution in the matrix of the steel and in a condition to precipitate when the steel is plastically deformed at boiler operating temperatures.

The heat treatment necessary to impart caustic resistance to a steel containing the proper metallic dispersive comprises exposure to a temperature exceeding 1100 F., followed by cooling.

In the case of these resistant steel s, this cooling may be accomplished by quenching provided the heat treatment temperature does not exceed the TAIL! IV Tensile strength' Percent reductionoiarca Percent elongation No R. T. 400 F. 475 F. 650F. R. T. 400' F. 475 F. 550 R. T. 400 F. 475 F. 550 F. 21:: $3 3312' St? $13 33: $8 $9 I 23:8 3:3 2:3 233% 5%? Thousand pounds per square inch. R. T.==room temperature.

Steels Nos. 6, '1 and a have been found to be resistant to caustic embrittlement when properly heat treated as described hereinafter. It will be observed from a comparison of the data of Table temperature range as compared with room temlower critical temperature as the developmentof martensite is to be prevented. However, cooling rates as fast as quenching are not necessary,

and in nickel-copper and nickel-aluminum steels, such as steels Nos. 6, 7 and 8, for example, air

- cooling from normalizing temperatures are adeperature values and this decrease was accompanied by only a moderate decrease in ductility.

Steels Nos. 6, 7 and 8 are further distinguished from steels Nos. 1, 2, 3, 4 and 5 by the ability to age harden, when properly heat treated, by dispersion of a metallic constituent containing an alloying element. The age hardening obtained by heat treatment which disperses a metallic constituent containing an alloying element is to be distinguished from blue brittle hardening where quate to retain the hardening phase sufficiently in solid solution to operate later. The thing to avoid is too slow cooling or too long an exposure at precipitation hardening temperatures such that the steel over-ages and the precipitate becomes too completely separated from solid solution and coalesced. Specific treatments which have been found satisfactory for nickel-copper steel are: v

(1) Normalizing from 1650 F.

(2) Normalize from 1650 F. plus aging at 900 F.; and" (3) Oil quenching from 1200 F.

Cold working after these conditioning heat treatments does not impair the resistance of these steels to caustic cracking.

a metalloid dispersive, such as an oxide, nitride or the like is precipitated. The blue brittle type of hardening is accompanied not only by the beforeimentioned decrease in ductility but also by low shock resistance after plastic straining and by low resistance to caustic cracking at boiler operating temperature. The steels free from or devoid of blue brittlenesswhlch' are hardened by dispersion of a'metallic' constituent containing an alloying element have higher tensile and impact strengthf when the metallic constituent precipitated or dispersed, but the ductility is relatively slightly affected, as compared with the strength and ductility of the steel in the unprecipitated condition.

For the purpose of giving those skilled in the art a better understanding of the invention and the benefits that may be derived from its application, data from typical tests are assembled in Tables V, VI and VII which are illustrative of the new results in safety and length of service life of' boilers embodying the present invention. In making the determinations of the stress above which caustic cracking develops, tests'were made 7 on tensile specimens in bombs containing activated caustic solution, i. e., fractional percentages of silicates and other salts which should be present with caustic to insure certainty of cracking. The tensile specimens had a minimum diameter of 0.200 inch at the longitudinal center and increased on either side along the circumference Satisfactory resistance to caustic embrittleof a circle having a ten inch radius and having From these data it will be apparent to those its center in the plane passing through the skilled in the art that the stress above which specimen at the point of minimum cross-section.

caustic cracking develops cannot be predicted from the tensile strength of the'steel at room..

. temperature, or at elevated temperature. A similar lack of relation exists between the yield;- point and the behavior of these steels to caustic embrittlement.

The double taper toward the center allowed a range of stress to be applied in each test to each specimen or bar.

From these data it will further be apparent to f plain carbon and silica-manganese steels such as Nos. 1 and 5 in the as rolled condition and No. in the normalized condition are subject to caustic cracking at relatively low stress; that the addition of nickel .15 to plain carbon steels produces improvement in resistance to caustic embrittlement in the as rolled condition (No. 2) and in the normalized condition (No. 3) that the addition of chromium i to plain carbon steels is of doubtful benefit (N0. Hemmtment 4); that low carbon nickel-copper steels in the annealed condition '(No. 9) or in the over-aged condition (No. 10) do not exhibit much, it any, improvement over nickel steels; that copper additions to plain carbon steels, even though the appreciably improve the resistance to caustic cracking (No. 17). On the other hand, low carbon nickel-copper steels similar to No. 9 and No.

10 when properly heat treated to place the metallic dispersive in solution and in condition to precipitate under plastic strain. at boiler operating temperatures exhibit reatly improved resistance to caustic cracking. Thus, steel No. 6,

in which the metallic dispersive is largely in so- 1650. F F steel is in a condition to age harden, do not no. 1650F,A 10

Per-

cent

TABLE V Percent 1. 86 0. 98 Annealed l. 86 0. 98 Slowly cooled 'sl Ni Composition Pcr- Per- Percent cant When a test was completed the bar was removed, sectioned longitudinally, polished and the polished section examined microscopically for cracks from the center up the taper toward'the l0 those'skilled. in the art that largest dimension. The diameter at the point where the last crack occurred was noted and the stress at this point was calculated and reported as the stress above which caustic cracking develops."

Steel No.

A. C.=air cooled.

C. R.=cold reduced.

0. Q.=oil quenched.

Percent elongation 5 a panache a iami...

475" F. 550 F. ar. 400F. 475%. 550 F.

. scacmcacn cements a mmmwnmum Tensile strength R. T. 400 F. 475 F. 550 F. R. T. 400 F.

lution, resists cracking up to a stress of 67,500 pounds per square inch as compared with 41,500 pounds per square inch for steel No. 9-an increase of over 50%. Similarly, steel No. 7 which has a chemical composition similar to steels Nos. 6 and 9 exhibited excellent resistance to caustic cracking when in the same condition as steel No. 5m 6. Steel No. 8 is also a dispersion hardenable steel in which 'aliiminum rather than copper is the alloying element that is or forms the metallic dispersive, and when the dispersive is largely in solution this steel also exhibits, high resistance to 5 caustic embrittlement. Steel No. 11 was partially aged prior to the test, yet it exhibited resistanceto caustic cracking comparable to steels Nos. 6 and 7 and greatly increased resistance as compared with the over aged steel No. 10.

0 Steel Nos. 12 and 13 were heat treated in the same manner as steels Nos. 6 and 11, respectively, and were then cold worked to reduce their cross-section about 10% prior to testing. The high resistance to caustic cracking exhibited by these cold worked steels is of great importance mmmmmmmmmm aaneaecaaacecacaa Steam pressure, lb. per

d pounds per square inch. room temperature.

TABLE VII Stool No.

Thousan R. '1.-

Strcss given in pounds per square inch.

Stresses above'which caustic cracking develops sition to steel No. 6. Steel No. 14 was given the same heat treatment as'steel No. 6 except that prior to the solution heat treatment it was given a grain coarsening treatment at 1850 F. to determine what, if any, relation exists between grain size and resistance to caustic cracking in nickel copper steels. Steel No. 16 was quenched in oil from 1200? F. The grain size ratios of steels, Nos. 6, l4 and 16 in the condition tested were about 1:2:4 respectively, yet all exhibited comparable high resistance to caustic cracking.

The foregoing data are typical of the results obtained with other steels that are free. from manifestations of blue brittleness and contain a dispersible or incompletely dispersed phase of a metallic constituent containing an alloying element. The base composition of the steel may be chosen in order to provide properties desirable for the particular application, ,e. g., grain size, texture of pearlite, strength of ferrite, hot workability, machinability, creep resistancaetc. The upper range of alloy content in the base steel is governed by .the necessity of maintaining a steel sot-t enoughjand ductile'enough to permit fashioning into boilers from rolled plate. Satisfactory results may be had with a steel chosen within the'following base composition:

Carbon up to about 0.50% Manganese up to about 2% Silicon up to about 1% Nickel up to about and optionally Molybdenum up to about 2% with other elements to the extent normally present in plate steels with a tensile strength not exceeding about 100,000 pounds per square inch. Chromium should preferably be less than 0.4% maximum.

With such a base steel metallic dispersion hardening component containing an alloying element should be additionally present. This alloying element may be chosen from the group consisting of:

Copper about 0.5 to about 3% Aluminum about 0.5 to about 3% Beryllium about 0.2 to about 3% Titanium about 0.5 to about 5% Preferably the base steel composition falls within-the following limits:

' Copper within the range of about 0.5% to about from about 0.5% to about 2.0% are preferred alloying elements for forming- 2.5% and aluminum the metallic dispersive. Specific examples within these ranges are found in the tables. Iron constitutes substantially the balance of the alloy, i. e., the steel contains no other elements in amounts suiilcient substantlally to change the parts not subjected to ;caused by internal hydrostatic pressures.

' in the formation of a Shell stress lb./in

properties imparted thereto by the above mencracks when loaded to 62,000 pounds per square inch at pounds per square inch steam pressure. At 500 pounds per square inch steam pressure, it developed no cracks when stressed below 67,500 pounds per square inch and at 850 pounds pressure it successfully withstood embrittlement up to 59,000 per square inch stress. The cracking stress of plain carbon steel; on the contrary, dropped from 50,000 pounds per square'inch to 29,000 pounds per square inch as the steam pressure increased from 150 to 500 pounds per square inch. The ability of the boilers embodying the present invention to operate safely and long at high pressures is of vast commercial value.

In order further to demonstrate the ability of steel No. 6 to withstand high temperature and pressure, several small pressure vessels were constructed. They were arranged so that a longitudinal tensile stress could be applied to the cylindrical shell in addition to the stresses They were fed with water containing caustic and tested to destruction. Typical results are given in Table VIII.

' TABLE VIII Steel No. 5

Temperature F ressure lb./in.

While we do not desire to be bound by any particular theory as to tlement and the manner in which the steels of the present invention resist caustic cracking and embrittlement, the progress of embrittlement appears in some way to be connected with galvanic action between the metal of the boiler and a.scale that forms on the surfaces exposed to the boiler water. At boiler operating temperatures a direct reaction between the iron of the metal and the boiler water is believed to occur which results scale of FeaOi adhering to the metal. This scale acts as a protective coating against further attack so long as it is intact,

but if it becomes broken or cracked so as to expose the metal, thereunder, a galvanic couple seems to be formed in which the scale is the cathode, the metal the anode and the boiler water the electrolyte. The progress of embrittlement is probably connected in some way with the concentration of caustic values in th'e boiler water in the neighborhood of such anodic points where localized intensification of strain may lead to cracking of the protective scale. The steels employed in the present invention have the ability to strengthen themselves automatically during use at points of local over-strain by precipitation of a metallic dispersive and prevent the occurrence of local areas that are highly stretched. Copper appears to be the alloying element best adapted to form a metallic dispersive capable of the mechanism of embritretarding caustic embrittlement. The said metallic dispersive does not precipitate appreciably at operating temperatures unlesss there is accompanying plastic strain. ,In the case of nickelcopper steels, for example, the metallic dispersive is readily precipitated at about 800 F. and above, but below about 750 F. substantially no aging takes place in the absence of plastic strain.

In the specification, and in certain of the appended claims, the characteristic property of the steels that resist caustic cracking has been described as the presence in the steels of a metallic dispersive containing an alloying element, the said dispersive being at least partially in solid solution but capable of precipiating at operating temperatu'reunder slight plasticstrain. Those skilled in the art will readily understand that the expression a steel having a metallic dispersive containing an alloying element describes a steel that is precipitation or age hardenable because of the presence of an alloying element such as copper, aluminum, beryllium, titanium, etc., which either alone or in combination with some other constituent of the steel forms a dispersible phase that may be put into or precipitated from 600% more than steel No. 16. This diiierence is believed to be the result of automatic strengthening of steel No. 16 due to the precipitation of the copper or copper containing dispersive at this temperature when under thisstrain,i.e.,plastically deformed by the stress applied. In steel No. 9 the copper dispersive was already largely or completely precipitated prior to the test and was therefore incapable of developing any further h'rrdness or strength under strain at operating temperatures.

The magnitude of the strain or deformation required to produce precipitation at boiler operating temperatures may differ as seen in curve A of Fig. 2. The extent of precipitation may vary with the magnitude of the strain. The term 'slight plastic strain should be interpreted to include the strains or deformations produced in solid solution by wellknown age hardening heat treatments. A characteristic property of precipitation hardenable steels is that, after cooling or quenching from an elevated temperature, they harden upon reheating above a certain minimum temperature. Ordinary steel, on the other hand, is hardest in a quenched state and loses its hardness to a greater or less degree upon reheating to the same temperature range in which the precipitation hardenable steels develop their hardness. 9

In th absence of strain, the minimum temperature to precipitate the metallic dispersive in substantial amounts from the steels of the present invention over a period of time corresponding to the expected service life of the pressure vessel, must lie above operating temperatures, i. e., above about 250 to 600 F. This is essential to long service life as the steel would otherwise become aged in normal use to such an extent as to lose most if not all of the resistance to caustic cracking imparted by the dispersible phase. But the dispersible constituent must also be capable of precipitating at operating temperatures of the boiler when the steel is plastically strained, i. e., plastically deformed as may occur around rivet holes, in rivets, expanded joints, heavily stressed members, in the metal adjacent cracks, etc. The ability to precipitate under such circumstances may be determined in a number of ways as those skilled in the art will readily understand. For example, stressing-strain curves obtained at operating temperatures will reveal the' strengthneing du to precipitation. This is illustrated in Fig, 2 for steels Nos. 9 and 16 which have substantially identical proportional limits at about 475 F. at which temperature the stress-strain curves were obtained in the customary manner well known to those skilled in the art. Curve A comparison which lay about 13,000 pounds per square inch above the proportional limit, steel No. -16 was elongated only about 0.00032" whereas steel No. 9 was elongated about 0.00198", or over boiler parts such as around rivet holes, in expanded joints, around cracks, etc. The ability to precipitate under slight plastic strain at operating temperature may be determined by subjecting a specimen of the steel to a stress above the proportional limit at operating temperature. If the curve obtained by plotting the stress-strain curve, as was done in Fig. 2, lies between the projection of the straight part of the curve and the curve obtained by similarly testing an over aged specimen of the same steel, 1. e., if it lies in the area between curve B and curve C of Fig. 2, the steel satisfies the requirement of precipitating under slight plastic strain at operating temperature as herein used.

Other terms used to designate the same state, e. g., that the dispersive is in a state of incomplete dispersion, or that the dispersive is partially or completely in solid solution, should be similarly understood.

In the claims defining iron as constituting substantially the balance of the steel, it is to be understood that elements not specifically recited which customarily are present in steels of the types contemplated by the present invention such as sulfur, silicon, manganese, carbon, phosphorus,

'etc., and which may be present as impurities or I intentional additions for purposes known to those skilled in the art, are not excluded from the scope of the claims.

Although the present invention has been described in connection with specific compositions, it is to be understood that they are given as illustrations, that other steels possessing the required a I iron constituting substantially the balance of the steel, said steel having been heat treated to place at least part of the copper in solid solution.

, 2. A steam boiler resistant to caustic cracking constructed at least in the parts vulnerable to caustic cracking of steel free from blue brittleness and containing carbon, the carbon not exceeding about 0.5%, manganese from about 0.20

to about 1.5%, silicon from about 0.05% to about 0.75%, nickel from about 0.3% to about 4.0%, copperfrom about 0.5% to about 2.5%, and iron constituting substantially the balance of the steel, said steelhaving a metallic dispersive containing copper in a state of incomplete dispersion.

3. A steam boiler resistant to caustic types of embrittlement constructed at least in the parts vulnerable to caustic embrittlement of steel free from blue brittleness and containing about 0.2% carbon, about 0.5% manganese, about 0.2% silicon, about 3% nickel, about 1.5% aluminum and about 94% iron, said steel having been heat treated to place at least part of the aluminum in solid solution.

4. A steam boiler resistant to caustic cracking constructed at least in the parts vulnerable to caustic cracking of steel free from blue brittleness and containing carbon, the carbon not exceeding about 0.5%, manganese from about 0.20 to about 1.5%, silicon from about 0.05% to about 0.75%, nickel from about 0.3% to about 4.0%, aluminum from about 0.5% to about 2%, and iron constituting substantially the balance of the steel,

said steel having a metallic dispersive containing aluminum in a state of incomplete dispersion.

5. A steam boiler resistant to caustic cracking constructed at least in part of steel free from blue brittleness and containing carbon, the carbon not exceeding about 0.5%, an efiective amount to about nickel, manganese, the manganese not exceeding about 2%, silicon, the silicon not exceeding about 1%, metal selectedfrom the group consisting of copper, aluminum, beryllium and titanium in. amounts from about 0.5% to about 3%, from about 0.5% to about 3%, from about 0.2% to about 3%, and from about 0.5% to about 5%, respectively, and iron constituting substantially the balance of the steel, said metal forming a metallic constituent at least partially in solid solution and capable of precipitating from solution under slight plastic strain at operating temperatures.

6. A steam pressure vessel resistant to caustic cracking constructed at least inparts vulnerable to caustic cracking of steel free from blue brittleness and containing carbon, the amount of carbon not exceeding about 0.5%, an effective amount to about 5 nickel, metal selected from the group consisting of copper, aluminum, beryllium and titanium in amounts from about.0,5% to about 3%, from about 0.5%v to about 3%, from about 0.2% to about 3%, and from about 0.5% to about 5%, respectively. and iron constituting substantially the balance of the steel, said metal formmg adispersible metallic constituent at least partially in solid solution.

7. In a steam pressure vessel, an element resistant to caustic cracking made of steel containing carbon, the amount otcarbon not exceeding about 0.5%, an efiective amount up to about 5% nickel, metal selected from the group consisting of copper, aluminum, beryllium and titanium in amounts from about 0.5% to about 3%, from about 0.5% to about 3%, from about 0.2% to about 3%, and from about 0.5% to about-5%, respectively, and iron constituting substantially the balance of the steel, said metal forming a dispersible metallic constitutent at least partially in solidsolution.

8. In a pressure vessel subjected to heat, an element resistant to caustic cracking made of steelc ontaining carbon,-the amount of carbon not exceeding about 0.5%, an effective amount up to about 5% nickel, metal selected from the groupconsisting of copper, aluminum, beryllium and titanium in amounts from about 0.5% to about 3%, from about 0.5% to about 3%, from about 0.2% to about 3%, and from about 0.5% to about 5%, respectively, and iron constituting substantially the balance of the steel, said metal forming a dispersible metallic constituent partially in solid solution, NORMAN B. FILLING.

FREDERICK G. STRAUB.

at least 

